Delocalization boosts charge separation in organic solar cells

Organic solar cells (OSCs) utilizing π-conjugated polymers have attracted widespread interest over the past three decades because of their potential advantages, including low weight, thin film flexibility, and low-cost manufacturing. However, their power conversion efficiency (PCE) has been far below that of inorganic analogs. Geminate recombination of charge transfer excitons is a major loss process in OSCs. This paper reviews our recent progress in using transient absorption spectroscopy to understand geminate recombination in bulk heterojunction OSCs, including the impact of polymer crystallinity on charge generation and dissociation mechanisms in nonfullerene acceptor-based OSCs. The first example of a high PCE with a small photon energy loss is also presented. The importance of delocalization of the charge wave function to suppress geminate recombination is highlighted by this focus review. When light is shined on semiconducting polymers, singlet excitons are promptly generated in organic solar cells. At a donor–acceptor heterojunction, excitons separate into holes on the donor and electrons on the acceptor as a result of the energetic offset of the molecular orbital. If the electron and hole separate further, they become free from Coulombic attraction and hence survive up to nano- or microseconds, long enough to be transported to each electrode. Otherwise, the geminate electron–hole pairs are likely to recombine to the ground state.